U.S. Pat. No. 3,899,671 to Stover discloses a transmitting station 10 which provides information to mobile-mounted receiving units 11 carried by vehicles. (Stover, col., 2, Ins. 1-38). The information transmitted may include information regarding turns, stop signs, signal lights, and distance to landmarks. (Id.) Stover discloses a warning symbol number of “18” in table 6 for “low clearance”. (Stover, col. 16, table 6). Table 6 provides possible selection of meanings assigned to symbols for written warnings. (Stover, col. 12, Ins. 34-36). Information concerning stop signs may be displayed on display 22 so that stop signs that are difficult to see will not be missed. (Stover, col. 4, Ins. 13-26).
U.S. Pat. No. 4,621,252 to Johns discloses a radio transmitter positioned adjacent a bridge to generate radio signals encoded with the height of bridge which is presented to vehicles. (Johns, abstract). A vehicle includes a radio receiver which decodes the radio signals and the height information. (Id.) A warning signal is produced if there is a danger that the vehicle cannot pass safely. (Id.)
U.S. Published Patent Application 2004/0201495 to Lim et. al. discloses an active transmitter in the vicinity of a horizontal overhead obstacle and a receiver unit in each automobile vehicle. (Lim et al., abstract). Visual and audio warnings are produced if a vehicle will not pass underneath an overpass. (Lim, pg. 4, paragraph 46).